Systems and Methods for Extraction of Carbon Dioxide From Air

ABSTRACT

The present invention describes methods and systems for extracting, capturing, reducing, storing, sequestering, or disposing of carbon dioxide (CO 2 ), particularly from the air. The CO 2  extraction methods and systems involve the use of chemical processes. Methods are also described for extracting and/or capturing CO 2  via exposing air containing carbon dioxide to a solution comprising a base—resulting in a basic solution which absorbs carbon dioxide and produces a carbonate solution. The solution is causticized and the temperature is increased to release carbon dioxide, followed by hydration of solid components to regenerate the base.

This patent disclosure contains material that is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent document or the patent disclosureas it appears in the U.S. Patent and Trademark Office patent file orrecords, but otherwise reserves any and all copyright rights.

All patents, patent applications and publications cited herein arehereby incorporated by reference in their entirety. The disclosures ofthese publications in their entireties are hereby incorporated byreference into this application in order to more fully describe thestate of the art as known to those skilled therein as of the date of theinvention described and claimed herein.

BACKGROUND OF THE INVENTION

The combustion of fossil fuels provides the vast majority of the globalenergy supply. The necessary byproduct of this combustion is carbondioxide (CO₂) gas. Accumulation of CO₂ gas in the atmosphere hasprovoked concern regarding its effect on the global climate and spawnedworldwide interest in the reduction of CO₂ emissions to the atmosphere.

One approach of managing atmospheric emissions is through a chemicalprocess known as air extraction, by which CO₂ is removed directly fromthe atmosphere.

The present invention presents a preliminary design for a plant thatused wet scrubber techniques to remove CO₂ directly from air.

SUMMARY OF THE INVENTION

The present invention relates generally to the field of extractors,including those that work to extract carbon dioxide from air. Thepresent invention relates to methods and devices for extracting carbonusing wet scrubbing techniques.

It is a general aspect of the present invention to provide new methodsor processes for extracting, reducing, capturing, disposing of,sequestering, or storing CO₂ or removing excess CO₂ from the air, aswell as new methods and processes for reducing, alleviating, oreliminating CO₂ in the air, and/or the emissions of CO₂ to the air.Another aspect of the invention relates to apparatuses, such as wind orair capture systems, to remove or extract CO₂ from air. As used herein,the term “air” refers to ambient air, rather than emitted gas, such asgas that is emitted from a smoke stack or an exhaust pipe. While thelatter may contain air, it is not typically considered ambient air. Inaccordance with the present invention, extraction of CO₂ from airinvolves source gas, which is at atmospheric temperature, pressure andambient concentration of CO₂.

One approach of managing atmospheric emissions is through a chemicalprocess known as air extraction, by which CO2 is removed directly fromthe atmosphere. This can be accomplished using wet scrubbing techniquesto extract CO2 from air then return the CO2 to a gaseous form afterseveral chemical transformations. The wet scrubbing is accomplished bycontacting a sodium hydroxide solution with the atmosphere. The chemicalabsorption of CO2 produces a solution of sodium carbonate, which is thencausticized using calcium hydroxide. The causticization processtransfers the carbonate ion from the sodium to the calcium cation andfrom the liquid to solid state. The product of this reaction is anemulsion of precipitated calcite (calcium carbonate) in a regeneratedsodium hydroxide solution. In order to perform the thermal decompositionof calcite (calcination), it is necessary to filter and dry the calcite.It is not necessary to dewater the calcite completely as some steam isrequired for the subsequent regeneration of calcium hydroxide. Theproduct of calcination is gaseous CO2 and calcium oxide (solid lime).Hydrating the calcium oxide to regenerate the calcium hydroxidecompletes the cycle.

The present invention is generally directed to methods and componentsthat can be utilized to design a plant comprising a self-containedsystem for extraction of CO₂ directly from air using wet scrubbingtechniques.

One aspect of the invention is directed to the implementation ofmechanical dewatering steps to reduce the water content of the wetcalcite before it enters the calciner. These steps comprise dryingschemes which incorporate the heat of calcium oxide hydroxylation intothe drying of the calcium carbonate precipitate. An example of a dryingscheme includes, but is not limited to, hot steam produced when theprocess is conducted at high temperatures raising the temperature of thecalcite filter cake and thus cause release of the remnant water as steamwhich is transported back to the slaking unit where it is used tohydroxylate calcium oxide.

In another aspect of the invention, the wet calcite filter cake issubjected to a mechanical dewatering step which includes the applicationof high frequency sonic energy to decrease the water content of thefilter cake.

The invention also provides for the use of liquid filtration aids toassist in calcite precipitate filtration and subsequent thermaldecomposition of calcite. Examples of liquid filtration aids include,but are not limited to, surfactants such as sodium salts of fatty acids.

The invention further provides for the use of solid filtration aids toassist in calcite precipitate filtration and subsequent thermaldecomposition of calcite. Examples of solid filtration aids include, butare not limited to, rice husks.

Another aspect of the invention comprises the use of a hollow screw headas a heat exchanger to preheat the combustion gases (natural gas andoxygen) prior to injection into the reaction vessel. This aspect isexemplified by, but not limited to, a screw comprising a hollowrectangular strip of metal placed at the opening in the bottom of thereaction vessel where falling calcium oxide particles will contact themetal resulting in heat transfer to the screw then to the combustiongases.

The invention also provides for a device designed for scrubbing a volumeof gas in a contained system comprising a laminar forced draft verticaltower with solid outside walls and interior vertical tubes through whichscrubbing liquid flows. In one aspect, the air flows concurrent to theliquid and air flow can be generated by fans or active or passive means.

The invention further provides for the use of passive energy sources tomove air through the forced draft system. For example, wind can beharnessed using a venture device to create a vacuum which draws air intothe tower of the forced draft system. In another example, solar energycan be used to heat a volume of air encased in a glass structureconnected to the top of the tower of the forced draft system. As the airin the glass structure is heated, the pressure inside the glassstructure rises and draws air through the tower. The invention is notlimited to these examples.

In yet another aspect, the invention uses a small scale coal fired heatsource to generate steam for power generation or to provide heat tocalcite particles for calcinations. In a non-limiting example, smalltubes filled with coal are equipped with an oxygen feed, a flame and anash collector where the flame ignites the oxygen and coal to maintain asteady temperature inside the tube.

In another of its aspects, the present invention provides a method ofcarbon capture that removes CO₂ from air. The method also advantageouslyserves to regenerate the sorbent employed in the method. The methodinvolves the use of an alkaline liquid sorbent, e.g., sodium hydroxide(NaOH)-based, to remove CO₂ from ambient air and produce carbonate ions.The resultant sodium carbonate (Na₂CO₃) solution is mixed or reactedwith calcium hydroxide (Ca(OH)₂) to produce sodium hydroxide and calciumcarbonate (CaCO₃) in a causticizing reaction, which transfers thecarbonate anion from the sodium to the calcium cation. The calciumcarbonate precipitates as calcite, leaving behind a regenerated sodiumhydroxide sorbent, thus regenerating the sorbent. The calciteprecipitate is dried, washed and thermally decomposed to produce lime(CaO) and gaseous CO₂ in a calcination process. Thereafter, the lime ishydrated (slaked) to regenerate the calcium hydroxide sorbent. In arelated aspect, this method can be implemented using air capturingsystems, for example, towers or air or wind capture units of variousdesign, which function as the physical sites where CO₂ is captured andremoved from the air.

In another aspect, the present invention provides a method forextracting or capturing carbon dioxide from air, comprising: (a)exposing air containing carbon dioxide to a solution comprising a base,resulting in a basic solution which absorbs carbon dioxide and producesa carbonate solution; (b) causticizing the carbonate solution with atitanate-containing reagent; (c) increasing the temperature of thesolution generated in step (b) to release carbon dioxide; and (d)hydrating solid components remaining from step (c) to regenerate thebase comprising step (a).

In another aspect, the present invention provides a method forextracting or capturing carbon dioxide from air comprising: (a) exposingair containing carbon dioxide to a solution comprising a base, thusresulting in a basic solution which absorbs carbon dioxide and producesa carbonate solution; (b) causticizing the carbonate solution with acalcium hydroxide containing reagent; (c) calcining the resultingcalcium carbonate under thermal conditions in which one or more mixedsolid oxide membranes is interposed between the combustion gases and theinput air; and (d) hydrating the product lime to regenerate the calciumhydroxide involved in step (b).

In yet another aspect the present invention provides systems andapparatuses for extracting, capturing, removing, or entraining CO₂ fromthe air. Such capture apparatuses can include wind and air capturesystems or a cooling-type tower for extracting, capturing, removing, orentraining CO₂ as further described herein. Fan driven systems are alsoencompassed.

In another embodiment, the present invention embraces methods andsystems for extracting CO₂ from the air using liquid sorbents.Accordingly, the invention provides a method of carbon capture thatremoves CO₂ from air using solid oxide membrane and liquid sorbents.Suitable sorbents include basic solutions, such as sodium hydroxide(NaOH) or potassium hydroxide (KOH), and other often viscous fluids,which are typically caustic. More specifically, the method involves theuse of a hydroxide-based, alkaline liquid sorbent, e.g., NaOH solutionsand the like, to absorb and remove CO₂ from ambient air and producecarbonate ions. The resultant sodium carbonate (Na₂CO₃) solution ismixed or reacted with calcium hydroxide (Ca(OH)₂) to produce sodiumhydroxide and calcium carbonate (CaCO₃) in a causticizing reaction,which transfers the carbonate anion from the sodium to the calciumcation. The calcium carbonate precipitates as calcite, leaving behind aregenerated sodium hydroxide sorbent, thus, regenerating the sorbent.The calcite precipitate is dried, washed and thermally decomposed toproduce lime (CaO) in a calcination process. The thermal decompositionis preferably performed to avoid mixing the CO₂ resulting form thecombustion process, providing the heat with ambient air. This embodimentuses solid oxide membranes to separate input air from the combustionprocess. Oxygen at elevated temperatures can pass through thesemembranes. After calcination, the lime is hydrated in a slaking process.In a related embodiment, this method can be implemented using aircapturing systems, for example, towers or air capture units of variousdesign, which function as the physical sites where CO₂ is captured andremoved from the air.

In yet another embodiment, the resultant NaOH is recycled using sodiumtri-titanate rather than calcium hydroxide. In this embodiment, thereaction occurs in a molten rather than in an aqueous state. As aresult, the absorption solution is highly caustic in order to minimizethe amount of water evaporation required. In another embodiment, theinvention encompasses a method for capturing carbon dioxide from air,comprising (a) exposing air containing carbon dioxide to a solutioncomprising a base resulting in a basic solution which absorbs carbondioxide and produces a carbonate solution; (b) causticizing thecarbonate solution with a titanate containing reagent; (c) increasingthe temperature of the solution generated in step (b) to release carbondioxide; and (d) hydrating solid components remaining from step (c) toregenerate the base comprising step (a). In one embodiment of thismethod, the base of step (a) is selected from sodium hydroxide, calciumhydroxide, or potassium hydroxide. In another embodiment, the carbonatesolution of step (a) of the method is a sodium carbonate (Na₂CO₃)solution. In another embodiment of the method, the solution of step (a)is causticized with sodium tri-titanate.

In related embodiments, the present invention provides systems andapparatuses for extracting, capturing, or entraining CO₂ from the air orwind. Such capture apparatuses can include a wind capture system or acooling-type tower for extracting, sequestering, or capturing CO₂ asfurther described herein. Fan driven systems are encompassed. Windcapture systems refer to freestanding objects similar in scale to a windenergy turbine. For example, such devices contain a pivot that ensuresthat contacting surface can follow the wind directions. The device canoperate with either liquid or solid CO₂ sorbents. A liquid based systemoperates using pumps at the base, which pump sorbent to the top of thedevice. Once at the top, the sorbent flows under gravity back to thebottom via a circulation system. The circulation system can encompasstroughs or other flow channels that expose the sorbent to air.Alternatively, the system could be vertical wires on which sorbent flowsfrom top to bottom. The device is sized such that the sorbent issaturated in one pass. A solid system contains moving components onwhich one or more solid sorbent is bound. These components aremechanically raised into the wind so as to absorb CO₂. Once saturated,the components are removed from the wind stream, isolated and strippedof CO₂. In another embodiment, a cooling tower contains a CO₂ removalzone in the air inlet at the base, which may contain either solid orliquid sorbents in a manner described above.

In another embodiment, a CO₂ capture system according to this inventioncan comprise filter systems wetted by a flow of sodium hydroxide thatreadily absorbs CO₂ from the air, and in the process, converts it tosodium carbonate. Without wishing to be bound by theory, if the pressuredrop across the system due to viscous drag is comparable to the kineticenergy density in the air, then the fraction of CO₂ removed from theflow stream becomes significant, so long as the sorbent materials arestrong absorbers. This is because the momentum transfer to the wallfollows essentially the same physical laws of diffusion as the carbondioxide transfer. In a cooling tower type of system, intake air ispulled through a filter system that is continuously wetted with sodiumhydroxide. Another type of system can involve a slight pressure dropgenerated by other means. In yet another system, air contacts sorbentsurfaces simply by the wind (or moving air) passing through the deviceor system. It will be appreciated that in the design of a contactsystem, the rate of absorption should be considered. In this regard, thevolume of sorbent per unit output of CO₂ is independent of the specificdetails of the air contacting design.

Advantageously, air extraction of CO₂ and systems for this purpose canbe sited based on site-specific conditions, which can includetemperature, wind, renewable energy potential, proximity to natural gas,proximity to sequestration site(s) and proximity to enhanced oilrecovery site(s). The system should be designed for ease of relocation.For example, the extractor may be sited at an oil field in order tominimize transport. In such a case, oil could be reformed and used inthe calcination system.

In other embodiments, chemical processes, e.g., calcinations andcalcining carbonate, are encompassed for the recovery of CO₂. Oneprocess involves oxygen blown calcinations of limestone with internalCO₂ capture. Such calcinations are carried out in a calcining furnacethat uses oxygen rather than air. The use of oxygen results in theproduct stream including only CO₂ and H₂O, which can be easilyseparated. In addition, power plant and air capture sorbent recovery canbe integrated into one facility. Another process involves electricallyheated calcinations. Yet another process involves solid oxide ionicmembranes and solid oxide fuel cell (SOFC)-based separation processes(e.g., Example 2). Another chemical process involves the electrochemicalseparation of CO₂ from Na₂CO₃, for example, using a three-chamberelectrolytic cell containing one cationic membrane and an anionicmembrane. The cationic membrane is located between the central cell andthe negative electrode while the anionic membrane is located between thecenter and the cathode. A current is applied to the cell and then sodiumcarbonate is introduced into the center cell. The ions move toward theopposite electrode. Hydrogen is evolved at the anode and oxygen gas isevolved at the cathode, resulting in the formation of NaOH at the anodeand carbon dioxide gas at the cathode. Several cells can be stackedtogether by placing a bipolar membrane at the electrode locations of thesingle cell. This serves to reduce the amount of gas evolved per unitreagent regenerated.

The present invention embraces remote CO₂ sequestration sites via aircapture. Such remote sequestration following the capture of CO₂ from aircan include ocean disposal from floating platforms or mineralsequestration in territories or environments having the appropriatemineral sites and deposits. The capture of CO₂ from air allows CO₂ to bedisposed of in remote areas that otherwise would be inaccessible to CO₂disposal due to the prohibitively high cost of transporting CO₂ toremote locations.

The present invention further encompasses CO₂ extraction from the oceanusing limestone and dolomite as sources of alkalinity. If provided withsufficient alkalinity, the ocean can remove carbon dioxide from the air.According to this embodiment, ocean disposal can be improved bycalcining limestone or dolomite to capture CO₂ from the air. During thisprocess, CO₂ is released to the air, but the resulting CO₂ uptake isnearly twice as large as the initial CO₂ emission. Thus, a metalhydroxide, e.g., magnesium or calcium hydroxide, dissolved into thesurface of the ocean will raise the alkalinity of the water leading tothe additional capture of two moles of CO₂ for every mole of CO₂ enteredinto the system. Illustratively, and without limitation, in solid form,an ion, such as a calcium ion, Ca⁺², can trap one CO₂ molecule in theform of CaCO₃. However, in dissolved form, the same ion can trap two CO₂molecules as bicarbonate ions (HCO₃ ⁻). Therefore, limestone that isheated (calcined) as described herein releases one CO₂ molecule, butwhen it is dissolved in the ocean, two bicarbonate ions are trapped. Inthis embodiment, the CO₂ that is dissolved in the mixed layer at the topof the ocean is kept in solution by the addition of calcium or magnesiumions. The mixed layer typically, but not necessarily, reaches a depth ofapproximately 100 m. Suitable sources of metal hydroxides include,without limitation, limestone, dolomite, or smaller deposits ofmagnesium carbonates. Although calcium carbonate is supersaturated insea water and is thus difficult to dissolve, sea water is still farbelow the point at which calcium carbonate spontaneously precipitatesout, thereby allowing for some increase in carbonate and/or calcium inthe surface waters of the ocean. Further, the total dissolved calcium inthe ocean is a quite large amount; therefore, the ocean is generallyinsensitive to additions that could allow for substantial increases instored CO₂. Magnesium carbonate also dissolves in sea water, but at aslower rate than does calcium carbonate. Also, the slow dissolution ofmagnesium carbonate can raise the carbonate ion concentration of seawater, which may be counterproductive to dissolving additionalcarbonate. Because added calcium ions disperse relatively rapidly uponexposure to the ocean surface, this can prevent a risk of precipitationof calcium carbonate into the ocean waters.

More specifically regarding this embodiment, a method is provided tointroduce alkalinity into the water as one or more metal hydroxides,e.g., without limitation, MgO/CaO; Mg(OH)₂/Ca(OH)₂; MgO/CaCO₃; orMg(OH)₂/CaCO₃, or a combination thereof. These metal hydroxides arecalcination products obtained by calcining a suitable starting calciumcarbonate- or magnesium carbonate-containing material, e.g., dolomite,limestone, or magnesite, at a temperature above about 400° C., or aboveabout 900° C. The resulting carbon dioxide is captured and sequesteredat the calcination site. For dolomite at a temperature above about 400°C., the CaO component is not calcined, while MgO is calcined at thisrelatively lower temperature. Calcination can be performed byconventional methods (e.g., Boynton, R. S., 1966, Chemistry andTechnology of Lime and Limestone, Interscience Publishers, New York, pp.3, 255, 258), or by using another energy source, such as solar energy,wind energy, electrical energy, nuclear energy, remote sites withunusable methane, etc. According to this method, the calcination productis finely dispersed into ocean or sea water by various procedures. Forexample, introduction into the water can occur from one or more ships orvessels that drag behind or between them a line that drops fine powderin the water. The size of the line is long enough so that localconcentrations of the material are not driven very high. Alternatively,the calcination products can be fashioned into larger pellets, asconventionally known in the art. The pellets are dropped or ejected intothe water, dissolve slowly and distribute the material relativelyuniformly over a larger area as they drift along. Pellets should containsufficient amounts of air, e.g., have sufficient air pockets, to float.Such pellets can advantageously be added to the water in largerquantities versus a fine dispersion. Of particular interest are pelletscomprising CaCO₃/MgO mixtures. By the practice of this method, the netCO₂ balance is positive, even if the CO₂ from the calcination is notcaptured. Notwithstanding, for every CO₂ molecule released by thismethod, nearly two CO₂ molecules are absorbed into the ocean, whichtakes up CO₂ from distributed sources.

In another embodiment, the present invention relates to methods oftransitioning from today's energy system comprising unsequestered CO₂resulting from the use of fossil fuels to the capture and disposal ofCO₂, and ultimately, to renewable energy with recycling of CO₂. Suchtransitioning methods comprise combining CO₂ capture with magnesiumsilicate disposal. In this embodiment, CO₂ can be removed from the air,but rather than disposing of the removed CO₂, it is used as a feedstockfor making new fuel. The energy for the fuel derives from a renewableenergy source or any other suitable source of energy that does notinvolve fossil fuels, such as hydroelectricity, nuclear energy. Forexample, CO₂ is initially collected and disposed of or sequestered inunderground deposits (such as in enhanced oil recovery, (EOR)) or inmineral sequestration. In this way, the source of the energy is fossilfuel that can be extracted from the ground. To maintain anenvironmentally acceptable material balance, the carbon must bere-sequestered or disposed of. Alternatively, carbon can be recycled asan energy carrier. Hydrocarbon, i.e., reduced carbon, contains energythat is removed by the consumer by oxidizing the carbon and thehydrogen, resulting in CO₂ and water. The capture of CO₂ from air allowsthe CO₂ to be recovered; thereafter, renewable energy can be used toconvert the CO₂ (and water) back into a new hydrocarbon. The productionof hydrocarbon can include a number of processes. Illustratively,Fischer Tropsch reactions are conventionally used to convert carbonmonoxide and hydrogen to liquid fuels, such as diesel and gasoline(e.g., Horvath I. T., Encyclopedia of Catalysis, Vol. 2, WileyInterscience, p. 42). Similar methods using CO₂ and hydrogen are alsoestablished. Hydrocarbon can be produced from CO₂ and hydrogen.Hydrocarbon production typically involves the use of energy, e.g.,electric energy, to convert water into hydrogen and oxygen, or CO₂ intoCO and oxygen. Thereafter, fuels such as methanol, diesel, gasoline,dimethyl-ether (DME), etc. can be made.

In other embodiments of this invention, CO₂ capture apparatuses andsystems are encompassed, especially for use in connection with thedescribed processes. In one embodiment, a wind capture system comprisesa CO₂ capture apparatus in which the air delivery system relies onnatural wind flow. Such a CO₂ capture apparatus can be situated in thesame or similar areas to those in which wind turbines are employed. Inanother embodiment, the invention embraces a water spray tower CO₂capture apparatus comprising a cylindrical tower, e.g., approximately100 feet in height, which is open to the air at its top and containsground level exit vents. A vertical pipe comprises the center of thetower through which water can be pumped; the pipe can be capped with anozzle that sprays water horizontally. Water is pumped to the top andsprayed into the air. The resultant evaporation creates a pocket of airthat is colder and denser than the air below it. This leads to a downdraft which forces air through the exit vents. The exit vents contain asolid or liquid sorbent for CO₂ capture. In another embodiment, theinvention embraces an air convection tower CO₂ capture apparatuscomprising a vertical cylindrical tower that is attached to a glassskirt situated approximately 1 foot above the ground level. The glassinsulates the air between the ground and itself, which raises the airtemperature. The hot air then exits through the central tower. A solidor liquid CO₂ capture device is contained in the tower. In anotherembodiment, the invention encompasses a CO₂ capture apparatus comprisinga glass covered slope, which comprises a glass sheet situated somedistance above ground level, e.g., between 0.3 m and 30 m, depending onthe size of the overall apparatus. The glass acts as an insulator thatcauses the air to heat in the sunshine and this results in a draft upthe hill. The resulting flow is guided over CO₂ absorber surfaces, whichremoves CO₂ from the air passing through it. In another embodiment, theinvention encompasses cooling towers to replace a conventional watercooling liquid with a liquid sorbent. The liquid sorbent evaporateswater; in addition, the liquid sorbent collects CO₂ in concentratedform. In all cases, the saturated sorbent is stripped of its CO₂ asdescribed herein.

In another embodiment, wind funneling devices are optimized forthroughput rather than air speed, thereby leading to optimization forCO₂ capture and sequestration. For example, air convection towersemployed for CO₂ capture can be shorter than towers designed forelectricity production, since increased height to promote air speed isnot a requisite for CO₂ sequestration. Further, in such CO₂ captureapparatuses, textile membranes are used to separate alkaline fluids fromthe open air. Such membranes comprise cloth-type fabrics that allow airpassage while limiting sorbent loss through spray. An illustrative, yetnonlimiting, fabric is Amoco 2019. Other CO₂ capture systems includethose that are adapted to wind flow, e.g., venturi flows that createsuction on a set of filters that are balanced by adjusting the size ofthe openings so as to maintain constant flow speed through thefiltration system. As an example, FIG. 2 shows a solid structure (blacklines) seen from above. As the air moves through the narrowed passage,the pressure drops (Venturi Effect). As a result, the higher pressureair inside the enclosures that are open to the back of the flow willhave a tendency to stream into the low pressure air flow. Openings arepreferably large in a high speed wind and small in a low speed wind inorder to maintain a constant pressure drop across the filter system andthus optimize the efficiency of the collector even in the face ofvariable wind speeds. By adjusting the size of the opening, e.g., usingshutters, baffles, etc., one can control the pressure drop across thefilter and can control the amount of air that emerges for optimized flowrates.

It will be appreciated that fans can comprise fan driven CO₂ captureapparatuses and systems, e.g., in CO₂ capture systems at the site of anoil well to perform EOR. The use of a fan or forced air system ensures aspecified air throughput, rather than having to rely on the fluctuationsof natural wind. By creating a constant air flow, a specified productioncan be achieved which may be desirable for production schemes thatrequire constant carbon dioxide output rates. The price for such anarrangement is higher energy cost and capital cost in the installationand operation of fans.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a schematic depiction of an overview of an airextraction process. Drying (d) and hydrating (h) are not specificallyshown. In accordance with an embodiment of the present invention, such aprocess is functionally integrated into an air capture system.

FIGS. 2A and 2B schematically depict a type of CO₂ capture system thatis adapted to wind flow, e.g., venturi flows. In FIGS. 2A and 2B, thethick black lines represent a solid structure as seen from above. As theair moves through the narrowed passage, the pressure drops (VenturiEffect). As a result the higher pressure air inside the enclosures thatare open to the back of the flow have a tendency to stream into the lowpressure air flow.

FIG. 3 is a wire diagram showing the process system method forextracting CO₂ from air.

DETAILED DESCRIPTION OF THE INVENTION Energy and Material Balance of AirExtraction

The combustion of fossil fuels provides the vast majority of the globalenergy supply. The necessary byproduct of this combustion is carbondioxide (CO₂) gas. Accumulation of CO₂ gas in the atmosphere hasprovoked concern regarding its effect on the global climate and spawnedworldwide interest in the reduction of CO₂ emissions to the atmosphere.This paper presents a preliminary plant design for removing CO₂ directlyfrom the atmosphere.

Existing studies have been performed which focused on the capture of CO₂from large stationary sources as the most promising “strong medicine”approach to carbon mitigation[1,2]. The captured CO₂ is compressed,dehydrated, and piped to a disposal site. Herzog concludes thatconventional power plants will incur costs of approximately $35 pertonne of CO₂ captured. More advanced systems, like the integratedgasification combined cycle (IGCC) and fuel cells, are expected to haveslightly lower capture costs. Capture will have to be followed bydisposal which will result in a total combined CO₂ mitigation costranging from $50-85 per tonne of CO₂ at the disposal site, raising thecost of electricity from 40 to 200%. Injecting the CO₂ into depleted oilor gas reservoirs and into deep coal seams to lower the cost ofproducing fossil fuels could offset the cost of mitigation.

The present invention is an alternative approach to managing atmosphericemissions through a chemical process, known as air extraction, by whichCO₂ is removed directly from the atmosphere. It can be estimated thatthe CO₂ capture potential and costs associated with an air extractionplant that produces one tonne per hour of CO₂. The process uses wetscrubbing techniques to extract the CO₂ and returns it to a gaseous formafter several chemical transformations. The wet scrubbing isaccomplished by contacting a sodium hydroxide solution (NaOH) with theatmosphere. The chemical absorption of CO₂ produces a solution of sodiumcarbonate (Na₂CO₃) which is then causticized using calcium hydroxide(Ca(OH)₂). The causticization process transfers the carbonate ion fromthe sodium to the calcium cation and from the liquid to solid state. Theproduct of this reaction is an emulsion of calcite (CaCO₃) in aregenerated NaOH solution. The calcite is filtered, washed, dried, andthermally decomposed by calcination, a common process used in the cementindustry for over 100 years [3]. The product of calcination is gaseousCO₂ and solid lime (CaO). Hydrating the lime to regenerate the calciumhydroxide completes the cycle. A detailed summary of the chemistry andchemical reactions involved in this process is provided elsewhere [4]. Asimplified overview of the system is presented in the drawings.

Process Overview

The CO₂ molecule undergoes a sequence of four chemical reactions as itpasses through the process. Each reaction will be considered in order tounderstand the mass and energy balances in a representative system. Thecomplete suite of reactions is shown below along with the enthalpies ofreaction given at standard atmospheric pressure and temperature.

2NaOH_((aq))+CO_(2(g))→Na₂CO_(3(aq))+H₂O_((l)) ΔH^(o)=−109.4 kJ/mol

Na₂CO_(3(aq))+Ca(OH)_(2(s))→2NaOH_((aq))+CaCO_(3(s)) ΔH^(o)=−5.3 kJ/mol

CaCO_(3(s))→CaO_((s))+CO_(2(g)) ΔH^(o)=+179.2 kJ/mol

CaO_((s))+H₂O_((l))→Ca(OH)_(2(s)) ΔH^(o)=−64.5 kJ/mol

The sum of the reaction enthalpies cited above is zero, as expectedgiven the cyclic nature of the process. A physical implementation ofthis process will require additional steps and contain inherentinefficiencies. As a thermodynamic minimum, the energy cost is−RTlog(P₁/P₂) where P₁ is the ambient partial pressure of CO₂ in air(375 ppm) and P₂ is the partial pressure of the output stream. Assumingambient temperature (300K) and an 80 bar output stream, we obtain aminimum penalty of 13.3 kJ/mol. The process may consume more energy.

The CO₂ content of air, at 375 ppm, is approximately 0.015 mol per cubicmeter (m³). In order to absorb 1 mol of CO₂, a minimum of 67 m³ of airmust contact the solution. The heat of formation of the reaction willraise the air temperature by ˜1.4 K. Some of the heat will also beabsorbed by the solution; if all of the heat were to go into solutionits temperature will rise by less than 0.1 K. As such the heat generatedthrough reaction (1) will not be recoverable. The small amount of heatgenerated by reaction (2) will also be lost to a negligible warming ofthe solution. The result is that reaction (3), the thermal decompositionof calcite, dominates the process and necessitates an external heatsource. It is highly desirable to recover as much heat from reaction (4)as possible as it can be used in the process. Summing up the enthalpiesof reactions (3) and (4) a minimum heat input of +124.7 kJ/mol can beobtained. This is the equivalent of 2.83 GJ per tonne of CO₂, which canbe compared to 2.05 GJ per tonne of CO₂ for monoethanolamine (MEA) [5].It should be noted that the temperature at which calcium hydroxidedehydrates is ˜700K [6]. Therefore, the energy recovered cannot be usedto drive reaction (3).

The calcite will be formed through precipitation, which will entrainwater from the solution. The drying of the precipitate will also requireenergy according to reaction (5) shown below.

H₂O_((l))→H₂O_((g)) ΔH_(vap)=+41 kJ/mol@373K,1 atm  (5)

The amount of water entrained with the calcite will be a function of thefiltration technology and not the amount of CO₂ captured. The netpenalty for the process must be overcome by the addition of heat from anexternal source. One method of obtaining this heat is through thecombustion of methane (CH₄) according to reaction (6).

CH_(4(g))+2O_(2(g))→CO_(2(g))+2H₂O_((g)) ΔH^(o)=−890.5 kJ/mol  (6)

In order to produce the required +179.2 kJ/mol a total of 0.20 mol ofmethane would be combusted producing an additional 0.20 mol of CO₂. Inan ideal situation, the combustion CO₂ would be captured andsequestered. This means that for every mole of CO₂ captured, a minimumof 1.20 moles needs to be compressed and sequestered. In practice,system inefficiencies and process requirements will require a greaterinput of heat thereby increasing the amount of combustion CO₂ generated.Coal may also be considered as a fuel source but would requireseparation from the lime in order to prevent fouling by the ash.

Absorption of CO₂ from Ambient Air

As stated above, the atmospheric concentration of CO₂ is approximately0.015 moles per cubic meter. The absorption of CO₂ into hydroxidesolutions has been studied [7] and the overall reaction found to be thefollowing:

CO_(2(g))+2OH⁻ _((aq))→CO₃ ⁻² _((aq)+H) ₂O_((l))  (7)

The rate of this equation is given as a constant multiplied by theproduct of the dissolved CO₂ and aqueous hydroxide concentrations. If weassume a well mixed, highly caustic solution then the hydroxideconcentration will be much larger than the dissolved CO₂ and thereaction will be pseudo-first order. Astarita provides an equation forthe CO₂ flux under these conditions which can be expressed in terms ofthe [OH⁻] concentration of the solution.

J_(CO) ₂ =√{square root over (D_(L)k_(d)b_(o)ρ′_(CO) ₂ )}  (8)

In this equation, D_(L) is the diffusivity of CO₂ in water; k_(d) is thekinetic constant; b_(o) is the concentration of hydroxide; ρ′_(CO2) isthe concentration of dissolved CO₂ at the surface. For a 1 mol/L NaOHsolution at 25° C. the maximum flux is 38 μmol/m² s. This value islarger than the published data [8] ranging from 3 to 10 μmol/m² s.Equation (8) shows the flux to be proportional to the square root ofhydroxide concentration. Experiments have shown it to reach a maximum2.5 mol/L NaOH [9]. At this latter concentration, the NaOH sorbent willhave a maximum CO₂ loading of 5.5% by mass or one third that of MEA[10].

It is worthwhile to note that the higher flux values obtained by Spectorand Dodge occurred when the packed tower was partially filled withpacking. This suggests that the wetted wall that would have existed inthe top half of the tower was more effective at removing CO₂ than thepacked section. Using equation (8) the maximum flux is estimated to be75 μmol/m² s for a 2.5 mol/L solution. By assuming the diffusion throughthe air side boundary layer follows Fick's Law, this can be assessedfurther. Using the conservation of mass we can equate the flux throughthe air and water boundary layers to obtain the following equation.

$\begin{matrix}{J_{{CO}_{2}} = { {D_{G}\frac{{\Delta\rho}_{{CO}_{2}}}{\delta}}\Rightarrow\delta  = \frac{D_{G}}{\sqrt{D_{L}k_{d}b_{o}}K_{H}}}} & (9)\end{matrix}$

In this equation, D_(G) is the diffusivity of CO₂ in air and K_(H) isHenry's constant. Substituting the values of 1 and 2.5 mol/L for theNaOH solution a boundary layer of thickness of 6 and 3 mm was obtainedrespectively. These thicknesses suggest that there is no advantage toturbulent flow. In fact, the larger pressure drop generated under theseconditions would increase the energy consumption of the process.

Sizing a capture system for a one tonne of CO₂ per hour facility theamount of surface area required is estimated. Given that one tonne ofCO₂ is 22730 moles, approximately 167,500 and 86,000 m² of surface areais needed for the case of 1 and 2.5 mol/L NaOH respectively. Contactinga solution with the atmosphere will result in evaporation. The amount ofevaporation can be controlled by increasing the ionic strength of theNaOH solution [11]. A more concentrated NaOH solution would reduce therequired contact area and the evaporative losses but it would require adilution step prior to causticization.

Causiticization

The aqueous transfer of the carbonate ion from the sodium to the calciumion is performed ubiquitously in the pulp and paper industry. Thereaction is limited by the concentration of the NaOH and in order toprecipitate calcite, according to reaction (2), the solution must belimited to 1 mol/L NaOH. If the caustic concentration is higher thanthis level, the calcite is unstable and will not precipitate [12]. Konnoalso noted that the size of the precipitate increases with temperaturefrom 5 μm to 15 μm as the solution temperature changes from 25 to 75° C.

The 1 mol/L ceiling provides a size for the reactor vessel for the 1tonne per hour plant. A one molar solution of NaOH would result in a 0.5mol/L solution of Na₂CO₃ and would therefore require 48 m³ of solution.Reaction (2) will progress to over 90% completion in one hour [13]. Inthe event that NaOH solutions stronger than 1 mol/L are used, or thereis significant evaporation, a dilution step will be required prior tocausticization. It is expected that thermal separation would be tooinefficient and some form of reverse osmosis is preferable.

Calcination

The end result of the causticization step is the production of calcite(CaCO₃) in a solution that is approximately 5% solids by mass. In orderto perform the thermal decomposition of reaction (3) it is necessary todry the solids. It is worthwhile to note that it is not necessary todewater the precipitate completely as some steam is required forreaction (4). One mole of water per mole of calcite results in a filtercake with a moisture content, mass of water over mass of solids, of 18%.Mechanical dewatering is less energy intensive than thermal drying andas such it is one embodiment of the invention to use this methodinitially. Vacuum filtration of calcite has been investigated, forsimilar particle sizes, and an irreducible moisture content of 19.5% wasobtained under a vacuum of 96 kPa [14]. It has also been found that theuse of ultrasonic energy aids dewatering [15]. In this case Singhobtained a 3% moisture content reduction for coal fines. The remainingwater will be evaporated for use in the hydration reaction. Steam dryingis an effective method that simplifies material handling and allows forenergy recovery. Hanson has investigated this option for the pulp andpaper industry and obtained 90% energy recovery [16].

The process of calcination is very mature and a wide variety of designsfor lime kilns are in use today [17]. These kilns range in efficiencyfrom 43 to 90%. The efficiency is defined as the theoretical heatrequirement (4.1 GJ/tonne) over the actual thermal input. The highestefficiency kiln is the parallel flow regenerative kiln which can reachefficiencies of 90% or higher [18]. It is rated for a particle size of10-30 mm. The flash calciner is the most efficient kiln rated forparticles in the micron range; it is rated at 70%. These kilns are firedusing air as the oxygen source. In order to maximize CO₂ capture, wewould use oxygen to fire the kiln and would expect the efficiency toincrease due to the absence of nitrogen. A preliminary estimate can beobtained by recognizing that a kiln with 70% efficiency will consume0.28 moles of CH₄ and therefore 1.15 moles of N₂ per mole of CO₂generated. Heating one mole of nitrogen takes 28 kJ of energy; thereforethe kiln will approximately consume an extra 730 MJ/tonne CO₂ for thispurpose. Pure oxygen would lower the thermal input from 5.86 to 5.13GJ/tonne and raise the efficiency from 70 to 80%.

This process involves several reactions in different phases and as sucheach reaction can be optimized separately. Given that the majority ofthe thermodynamic penalty is associated with the calcination reaction,it is desirable to minimize this step. Ongoing research is suggestingthat solar ovens may be able to replace a portion of the calcination.Meier et al. have obtained 98% calcination in solar thermal reactors atproduction rates of kilograms per hour [19]. This technology would haveto increase in production by two orders or magnitude and trap the CO₂generated prior to being included for air extraction.

Oxygen Production

The use of combustion energy to drive the calcination reaction willresult in the generation of CO₂. In order to simplify the capture of theCO₂ generated in the kiln, firing the kiln with oxygen is proposed. Thiswill create a flue gas stream consisting mainly of CO₂ and steam.Currently the industry standard for oxygen production is cryogenicseparation. Much interest is focused on the development of hightemperature oxygen separation using ion transport membranes (ITMs). ITMsoperate at temperatures around 800-900° C. and offer cost reductions of30% compared to cryogenic separation [20].

The real value in the ITM technology is the synergy with air extraction.ITMs generate an overall product mix of oxygen, power and steam [21].All of which are needed for air extraction. The oxygen is released at˜280° C. which will conserve 82 MJ/tonne of CO₂ versus cryogenic oxygen.This is not significant compared to the overall energy consumption,however, if the nitrogen off gas is released at the same temperaturethen 600 MJ of low grade heat are available. This is 66% of the thermalenergy required to dry the calcite. The ITM will consume both power andmethane. Assuming half of the oxygen is removed from the air stream,then 0.2 moles of methane is required per mole of CO₂ to combust theremaining oxygen. This is approximately equal to the amount of methanerequired to heat the air to firing temperature, 1400° C. Assuming a 40%conversion factor, the turbine will generate approximately 480 kW ofpower. This system would process ˜2,400 m³ of air, significantly lessthan the volume of air required to remove 1 tonne of CO₂ per hour. Ifthe turbine exhaust is mixed with the air extraction intake, the partialpressure of CO₂ can be raised from 375 to 450 ppm. This will increasethe absorption rate and decrease the surface area needed. The resultantturbine CO₂ can also be mitigated by a separate post combustion MEAsystem.

The exhaust from the turbine may even provide a portion of the drivingforce for the airflow through the contactor, further reducing powerconsumption. It is expected that the use of ITM technology willeliminate the need to import power while providing further energysavings by combining two high temperature reactions, oxygen separationand calcination. In this aspect of the invention, the process will usemore methane which exposes air extraction even further to pricefluctuations.

Preliminary Cost Estimate

Any cost estimate for a novel process is going to contain uncertainties.There is still, however, value in performing the analysis. A firstattempt at considering all of the financial charges will highlight whichcosts will dominate and where further analysis is required. It is alsouseful to establish a process design. Once a preliminary design isestablished it can be review by others in the field and anyexternalities overlooked will be brought to light.

The dominant chemical reaction in this process is the calcination of thelimestone according to reaction (3). Keith and Ha-Duong suggest that thecapitol cost for a calciner is around $1,000 dollars for each kg CO₂/hrof capacity [22]. A one tonne per hour plant would require a capitalinvestment of 1 million dollars for the calciner.

The wet scrubbing will require contact surface which can be estimatedfrom the flux rate calculated above. Using a 2.5 mol/L NaOH solution wewill require ˜86,000 m² of surface. Using a commercial supplier such asMcMaster-Carr® we can purchase 4′×8′ polypropylene sheets, 1.6 mm thick,for $23/sheet and a total charge of $330,000. Blowers will be requiredto move the air. If we extract 66% of the CO₂ from the air stream wereach a total throughput of 2.30 million cubic meters of air per tonneof CO₂ captured. A 30″ direct drive tube-axial fan from the Grainger®2001-2002 catalog is rated for 25,125 m³/hr and consumes 2 kW of powerwhile retailing for $1,600. This rating is for negligible pressure head.This process would require 92 fans consuming a total 184 kW of power or0.662 GJ of energy to capture one tonne of CO₂ in one hour. This is theequivalent of 16% of the calcination energy, reaction (3), andhighlights the significant advantage of natural airflows.

Regardless of the method of filtration, the solution will have to bepumped from the capture site to the precipitation reactor. Assuming thatthe reaction rate is sufficient for the calciner we can estimate thepumping power required by calculating the energy needed to overcomegravity. Such a calculation would take the form shown below.

${{Power}({kW})} = {\frac{1}{e}*( \frac{\rho \; {Qh}}{367} )*\frac{1\mspace{14mu} {kW}}{1000\mspace{20mu} W}}$

If assume a one story pumping height (4 m) is assumed, using the densityof a sodium carbonate solution of 1050 kg/m³ reported in the CRCHandbook (p 8-77), and an 80% pump efficiency (e), this equation can besolved. The pumping power requirement would be 640 W which is equivalentto a 2.32 MJ energy penalty to capture a tonne of CO₂ in one hour. Thisis a negligible amount in comparison to the calcination energy andpumping will not contribute significantly to the overall energyrequirements. A wet scrubbing technique to capture the CO₂ will requireadditional pumping of the sodium hydroxide solution. For simplicity ofcalculation it can be assumed that this pumping is equivalent to theenergy required to transfer the saturated sorbent to the causticizingvessel. The same battery of pumps would likely be used for both tasksand an energy penalty of 2.32 MJ per tonne of CO₂ per hour will beincluded.

Grainger® also retails an aluminum chemical transfer pump which moves 21gallons per minute (gpm) at 4 m head. The required pumping is 45 m³/hror 200 gpm thus necessitating 10 pumps. Each pump retails for $280 andso the battery of pumps would cost ˜$3,000. The plant will also requireholding tanks to contain the solution during the causticizationreaction. Using McMaster-Carr again we obtain a charge of $17,000 for 23tanks with 560 gallon capacity at $740/tank.

The filtration of the precipitate will require a vacuum filter. Thistechnology is highly process specific and this invention includes only arepresentative figure. A charge of $100,000 has been included based onprices posted on the internet and an energy penalty of 0.25 GJ/tonne CO₂based on a laboratory vacuum filter. With the exception of the calciner,the costs listed above represent the equipment alone. In order to closerapproximate an actual plant one can multiply these costs by a factor of2.98. This factor is suggested in Perry's Handbook (p. 9-72) andrepresents the additional cost of equipment installation, piping,electrical, instruments, battery limit building and service, excavationand site preparation, and auxiliaries [23].

Prior to injection into the ground the CO₂ will have to be compressed,which will consume energy. Blok et al. give a figure of 281 kJ_((e))/kgfor compression to 80 bar, which will be used in this analysis [24].Blok et al also list the cost for CO₂ compression as $61,200/(ton/hr)installed. The oxygen cost is taken as $27 per tonne of O₂ with anenergy consumption of 220 kWh/tonne [5].

The table below compares the individual and net energy penalties to thetotal penalty.

TABLE 1 Energy Metrics for 1 tonne/hr Air Extraction Plant Percent ofEnergy penalty Total (7.66 Item (GJ/tonne) GJ/tonne) Blowers for Air0.662 9% Sorbent 0.0023 0% Recirculation Pump to 0.0023 0%Causticization Filtration of 0.25 3% Precipitate Drying of 1.036 14% Precipitate Heat for Calcination 5.13 67%  Cryogenic Oxygen 0.242 3% CO₂Compression 0.336 4% Heat Recovery - (2.39) (31%)  Hydrating Energy 5.2768%  Consumption

It is also important to note that the total electrical requirement, i.e.blowers, pumps, oxygen, and compressors, comprise approximately 20% ofthe total energy penalty. The total power requirement is 415 kW.

The capital costs are assumed to be amortized at 10% over 20 years.Given this information a preliminary cost estimate for the airextraction plant can be performed. As a base case cryogenic oxygen isused, a 2.5 mol/L NaOH solution and a methane fired calciner that is 80%efficient. A filter cake moisture content of 20% can be used and thegoing market rate for electricity of 4.30/kWh with a multiplier of 1.5to include carbon capture and storage costs. The going market rates formethane, coal, and electricity were obtained from the NYMEX exchange.

TABLE 2 Cost per tonne of CO₂ captured - Base Case Delivered StrandedCoal Gas Gas $60.5/tonne Costs $6.80/MMBtu $3.00/MMBtu tce Capital 38.5438.54 38.41 Fuel 46.75 26.19 27.39 Electricity 28.32 28.32 33.78 Total113.61 93.05 99.58 Per gallon 1.04 0.85 0.91 gasoline

This data is used to project the effect of process improvements on thecost per tonne of CO₂. The effect of increasing the efficiency of thecalciner to 90% and decreasing the efficiency of the filtrationequipment is investigated. The filter cake is assumed to have a moisturecontent of 25%.

TABLE 3 Cost per tonne of CO₂ captured - Process Efficiency DeliveredImproved High Gas Calciner Moisture Costs $6.80/MMBtu 90% efficient 25%cake Capital 38.54 38.54 38.54 Fuel 46.75 41.47 46.75 Electricity 28.3227.66 28.32 Total 113.61 107.66 113.61 Per gallon 1.04 0.99 1.04gasoline

It is interesting to note that the increased moisture had no effect onthe cost. This is because with the steam hydration of reaction (4) thereis enough low grade heat to evaporate the water.

The effect of introducing novel technologies into the process can alsobe projected. Specifically the introduction of solar thermal ovens andITMs is investigated. The solar thermal oven will have the effect ofreducing the fuel consumption significantly, it is assumed that 50% ofthe calcite is decomposed using this technique. As described above, theITMs will alter the process significantly. The power produced by theturbine will be used by the plant and as a result a cost reduction onthe oxygen is not taken. The use of ITMs will significantly increase thefuel consumption and at $6.80/MMBtu is similar in cost to the base case.As such, the ITMs can be combined with stranded gas and a highefficiency kiln. The final column will present a best case scenariowhere stranded gas is used as fuel in conjunction with a high efficiencycalciner, ITMs and solar ovens.

TABLE 4 Cost per tonne of CO₂ captured - Novel Process Delivered SolarBest Gas Oven ITMs Case $6.80/ 50% Stranded All four Costs MMBtucalcined gas conditions Capital 38.54 38.54 36.21 36.21 Fuel 46.75 23.3833.33 21.72 Electricity 28.32 25.38 0.00 0.00 Total 113.61 87.29 69.5457.93 Per gallon 1.04 0.80 0.64 0.53 gasoline

This study is meant to provide a first attempt at estimating the cost ofa large-scale facility designed specifically to extract CO₂ directlyfrom the atmosphere. Given the assumptions made air extraction isexpected to be competitive with the CO₂ capture technologies currentlybeing proposed. More importantly it is unlikely that retrofittingexisting power plants will result in 100% CO₂ recovery, meaning thatsome additional CO₂ recovery will be necessary even for MEA systems.

There are three general conclusions that can be drawn from this paper.First, the cost of methane is significant for the system proposed.Therefore buying methane from the open market may be prohibitive andother fuels and/or stranded gas would be better sources. In cases wherethe cost of methane is in the vicinity of $4/MMBtu then this system canbe considered. Second, the system is scaled to 1 tonne per hour and anyincrease in size would likely result in cost savings. Finally, thecapital cost of the plant carries a large amount of uncertainty, as thetechnology is novel. As these systems move towards implementation lowerproduction costs can be expected. It is worthwhile to investigate otheroptions for generating a combustion off-gas stream that contains onlyCO₂ and steam. These options include; solid oxide fuel cells and/orother indirect heating systems. It is expected that the system presentedhere would be one of the more expensive ones.

The most important conclusion to be drawn from this work is that theprocess warrants further investigation. Each step of the process hasbeen studied and some cases have been used for over a century. Theenergy penalty is also concentrated in one step. This provides asuitable focus for improvements in efficiency. As such, optimization forair extraction purposes should be possible. Furthermore, air extraction,even as a small portion of the CO₂ mitigation portfolio can have apowerful influence because it is the only industrial CO₂ capturetechnology that can directly affect atmospheric levels. It may alsoprovide an effective alternative to fuel switching. The separation ofcapture from generation will allow for the optimization of each processindividually. This in turn should result in a more efficient use of theprimary energy source. In the end, this is the objective of any CO₂mitigation program. In the event that fuel prices reach levels where gasto liquid processes for hydrocarbon production are economical, airextraction can provide suitable feedstock from depleted gas fields.

REFERENCES

-   1. Herzog H. J., D. E. M., Carbon dioxide recovery and disposal from    large energy systems. Annu. Rev. Energy Environ., 1996. 21: p.    145-166.-   2. White C. M., S. B. R., Granite E. J., Hoffman J. S., Pennline H.    W., Separation and capture of CO₂ from large stationary sources and    sequestration in geologi formations. J. of the Air & Waste    Management Association, 2003. 55: p. 645-715.-   3. Boynton, R. S., Chemistry and Technology of Lime and Limestone.    1966, New York: Interscience Publishers.-   4. Zeman F. S, L. K. S., Capturing carbon dioxide directly from the    atmosphere. World Resource Review, 2004. 16(2): p. 157-172.-   5. Herzog, H. J., Assessing the Feasibility of Capturing CO₂ from    the Air. 2003, Massachusetts Institute of Technology: Boston.-   6. Zsako J., H. M., Use of thermal analysis in the study of sodium    carbonate causticization by means of dolomitic lime. Journal of    Thermal Analysis, 1998. 53: p. 323-331.-   7. Astarita, G., Mass Transfer with Chemical Reaction. 1967,    Amsterdam: Elsevier Publishing Company. 176.-   8. Spector N. A., D. B. F., Removal of carbon dioxide from    atmospheric air. Trans. Am. Inst. Chem. Engrs., 1946. 42: p.    827-848.-   9. Tepe J. B., D. B. F., Absorption of carbon dioxide by sodium    hydroxide solutions in a packed column. Trans. Am. Inst. Chem.    Engrs., 1943. 39: p. 255-276.-   10. Desideri U., P. A., Performance modelling of a carbon dioxide    removal system for power plants. Energy Conversion and    Management, 1999. 40: p. 1899-1915.-   11. Olsen J., J. A., Aly G., Thermophysical properties of aqueous    NaOH—H2O solutions at high concentrations. International Journal of    Thermophysics, 1997. 18(3): p. 779-793.-   12. Konno H., Y. N., Kitamura M., Crystallization of aragonite in    the causticizing reaction. Powder Technology, 2002. 123: p. 33-39.-   13. Dotson B. E., K. A. Causticizing reaction kinetics. in 1990    Tappi Pulping Conference. 1990: Tappi Press.-   14. Besra L., S. D. K., Roy S. K., particle characteristics and    their influence on dewatering of kaolin, calcite and quartz    suspensions. Int. J. Miner. Process., 2000. 59: p. 89-112.-   15. B. P., S., Ultrasonically assisted rapid solid-liquid separation    of fine clean coal particles. Minerals Engineering, 1999. 12(4): p.    437-443.-   16. Hanson C., T. H., Steam drying and fluidized bed calcination of    lime mud. Tappi Journal, 1993. 76(11): p. 181-188.-   17. Oates, J. A. H., Lime and Limestone: chemistry and technology,    production and uses. 1998, New York: Weinheim: Wiley-VCH.-   18. Cella, G. M., The TWIN-D lime shaft kiln—a new generation. ZKG    International, 1995. 48(12): p. 644-650.-   19. Meier A., B. E., Cella C. M., Lipinski W., Wuillemin D., Palumbo    R., Design and experimental investigation of a horizontal rotary    reactor for the solar thermal production of lime. Energy, 2004.    29: p. 811-821.-   20. Dillon D. J., P. R. S., Wall R. A., Allam R. J., White V.,    Gibbins J., Haines M. R. Oxy-combustion processes for CO2 capture    from advanced supercritical PF and NGCC power plant, in Greenhouse    Gas Control Technologies 7. 2004. Vancouver, Canada.-   21. Allam R. J, M. C. J., White V., Stein V., Simmonds M. Oxyfuel    conversion of refinery process equipment utilising flue gas recycle    for CO2 capture. in Greenhouse Gas Control Technologies 7. 2004.    Vancouver, Canada.-   22. Keith D. W., H.-D. M. CO2 capture from the air: technology    assessment and implication for climate policy. in Greenhouse Gas    Control Technologies 6. 2002. Kyoto, Japan: Pergamon.-   23. Perry R. H., G. D. W., ed. Perry's Chemical Engineers' Handbook.    7th ed. 1997, McGraw-Hill: New York.-   24. Blok K., W. R. H., Katofsky R. E, Hendriks C. A., Hydrogen    production from natural gas, sequestration of recovered CO2 in    depleted gas wells and enhanced natural gas recovery. Energy, 1997.    22(2/3): p. 161-168.

Additional Embodiments

Drying the calcite precipitate: One step in the air extraction processas discussed before is the refreshing of the sodium hydroxide sorbentsolution. After this sodium hydroxide solution has passed through theair extractor unit it has become enriched in sodium carbonate. Byletting the sodium carbonate react with calcium hydroxide, in a processshaped after the Kraft process, the sodium carbonate is turned back intosodium hydroxide solution leaving behind a calcium carbonate (calcite)precipitate. This precipitate is formed in an aqueous suspension fromwhich it needs to be separated. Thickeners may provide one approach toreduce the liquid content, but fine suspensions have a tendency to holda large amount of water. Wet calcite should not enter the calciner asthe energy penalty for driving off the steam in the calciner would bevery large and reduce the overall efficiency of the process. Therefore,the invention discussed here adds mechanical dewatering steps into theprocess flow diagram in order to reduce the water content as much aspossible. A first such step would involve the filtration of the calciteproducing a wet filter cake. Depending on the details of the downstreamprocessing it may prove advantageous to wash the filtrate to remove mostof the remaining sodium hydroxide, but regardless of this step, the nextsteps will involve mechanically dewatering of the calcite filter cake.

It is noted, however, that the calcium oxide formed in the calcinerneeds to be converted to calcium hydroxide by adding water that couldeasily be derived from the wet calcite. (This process will be referredto as slaking even if it is performed in a gas solid reaction of limeand steam.) One part of the invention is to use mechanical dewateringsteps that ideally reach a water content of 15% in the calcite material,followed by a thermal drying process that uses heat from the reactionCaO+H₂O→Ca(OH)₂+heat. In an optimal design this process will beperformed at elevated temperatures were the water is present as steam.The heat of the reaction would be transferred by methods known to thosepracticed in the art to the wet calcite. The hot steam will raise thetemperature of the filter cake and thus cause the release of the remnantwater as steam which is transported back to the slaking unit. Oneapproach is to use heat exchangers between the fluidized bed performingthe slaking the other is to circulate hot steam between the two beds.Some of this steam will be consumed in the slaking process the rest isused to carry the heat of the process away. The hot steam by heating thewater in the calcite filter cake would drive the production ofadditional steam which ideally is just sufficient to replace theconsumed water. In practice, the filter cake may prove to be too dry, inwhich case make up water has to be added to the cycle, or too wet inwhich case some of the excess steam must be released and condensed out.The slaking process releases an amount of heat sufficient to boil offabout 2.5 moles of water.

The heat contact could be achieved by hot gas streaming over and throughthe filter cake, or indirectly by heating the surfaces on which the wetfilter cake rests. Practical implementations may utilize both forms ofheating. Heat transfer to hot surfaces for wet filter cake may simplifythe heat transfer. But once the material is sufficiently dry,entrainment in a gaseous flow may prove advantageous. This latterapproach reduces the need for grinding up the dried up filter cake. Ofcourse it is also possible to use lower grade heat that is available inthe heating and cooling steps of the process for the drying steps andconserve the high quality heat that is generated in the slaker for otherprocesses, including steam generation for electricity production.Roughly, the heat of hydroxylation from steam is sufficient to convert2.5 moles of water into steam.

A variation of this method which also relies on the slaking operation todrive the drying of the calcite would be to use the dry lime as a dryingagent as it is strongly hygroscopic until it is converted to calciumhydroxide. In such an implementation it is important, however, that thedewatering step reduced the water content of the wet calcite to lessthan one mole of water per mole of limestone or to a moisture content ofless than 15% by weight.

Specific implementations of the calcium carbonate/calcium hydroxidecycle are outlined that amplified on drying schemes that incorporate theheat of hydroxylation into the drying of the calcium carbonateprecipitate.

Use of sonic energy to assist precipitate filtration. The followingdiscussion amplifies on the implementation of the mechanical dewateringstep of the process. Since Mechanical dewatering steps are far moreenergy efficient than thermal drying operations. The optimum watercontent of the filter cake is about 15% moisture content by weight.Steam drying or other approaches of removing the remnant water from thesystem. The filter cake is dewatered either by pressing or movingthrough a filtration step to minimize its liquid content. If so desiredit is washed with water to remove sodium hydroxide. The remainingmoisture is then removed once more. Besra et al. state that using vacuumfiltration one can expect to produce a minimum water content of ˜20% byweight. This suggests that for every mole of water precipitated outthere are approximately 1.4 moles of water that need to be removed bydrying. This part of the invention concerns itself with methods to drivethe remnant water content down to 15%. For this purpose filtration iscombined with the application of high frequency sonic energy. This canreduce the water content of a filter cake by several percent. The novelaspect of this invention is to use sonication in calcite filtration toadjust the water content of the calcite precipitate to 15%. The deliveryof sonic energy is adjusted to maintain constant moisture content.

Use of surfactants to assist precipitate filtration with subsequentcombustion. Similarly, the addition of surfactants to the filter cakehas been found to reduce water content in coal fines and is expectedwork as well with other materials. Surfactants are a group of moleculesthat consist of long carbon/hydrogen chains in which one end has aslight charge and different molecular components. Once the calcite hasbeen filtered it is dried and then heated to ˜900 degrees Celsius inorder to induce thermal decomposition. In the implementation envisionedhere oxygen and fuel are mixed with the calcite to provide the heat ofcalcination. Therefore the surfactant molecules will act as additionalfuel and release their energy through combustion processes. The releaseof energy will serve to reduce the additional fuel energy required tothermally decompose the calcite. One novel aspect of the invention isthe use of surfactants in calcite filtration and subsequent combustion.Effective surfactants should be of low cost and be limited to chemicalconstituents that do not create harmful combustion products.Illustrative examples are sodium salts of fatty acids (soaps).

Use of rice husks or other suitable biomass for filtration withsubsequent combustion. There are a number of filtration aids that addedto the precipitate prevent its complete agglomeration and thus maintainpathways for the water to drain out during filtration. A knownfiltration aid of this type are rice husks. Rice husks would mix in withthe calcite, they would be combusted in the calcination step and thusprovide a fraction of the energy input in a carbon neutral manner. Thistechnique is similar to 3) with the exception that solids are used asfiltration aids rather than surfactants which change the surface wettingproperties in the solid liquid interaction. An additional considerationis that these rice husks would introduce a biomass fuel component in thecalcination step. Since the CO2 from this combustion step is captured,together with the CO2 that is freed from the limestone, the net effectis an additional capture of CO2 from air via the photosynthesis thatcreated the rice husk. One novel aspect of the invention is the use ofrice husks in calcite filtration and calcination.

Hollow screw heat exchanger to heat natural gas. The thermaldecomposition of the calcite is expected to occur at temperatures at orabove 900 degrees Celsius. As such, the combustion gases, oxygen andnatural gas will require preheating in order to maximize efficiency. Onepotential method is to use the heat contained in the calcium oxideleaving the reaction vessel. It is anticipated that the vessel will besimilar to a cyclone use for gas cleaning. In this configuration, thesolid particles leave through an opening in the bottom of the vessel.This invention proposes placing a vertical screw at this location tocontact the falling calcium oxide particles. The contact between thecalcium oxide and the screw will result in the transfer of heat to thescrew. This invention also proposes to use the natural gas and oxygen asseparate cooling fluids in a counter current system. These fluids willheat up to the reaction temperature prior to be injected into the vesselfor combustion. The calcium oxide will be cooled from 1300 to 600degrees Kelvin yielding approximated 37 kJ of heat per mol of calciumoxide.

The feasibility of such a device will be evaluated by calculating thesize of a potential screw. The simplest design is one where the screwconsists of a hollow rectangular strip of metal. For this analysis it isassumed the opening in the screw is 25 cm long and 10 cm high with a 2.5cm cast iron wall. The CRC handbook lists a thermal conductivity of ironof 34 W/m K at 973 degrees Kelvin.

This analysis is based on a 1 tonne of CO₂ per hour air extractionplant. In this case a mass flow of 1285.2, 345.6 and 86.4 kg/hr forcalcium oxide, oxygen and methane are expected, respectively. This canalso be expressed in molar flows of 22.95, 10.8 and 5.4 kmol/hr. Usingavailable heat capacity data one can calculate that the calcium oxidewill release 850 MJ of heat, the oxygen and methane will absorb 466 and352 MJ respectively. This means there is a slight excess of heatconsisting of 32 MJ contained in the calcium oxide.

The CRC Handbook also lists the densities of methane and oxygen as 0.02mol/L at 600K and 0.03 mol/L at 380 K respectively. Using thesedensities the mass flows can be converted to volumetric flows and findthat the methane will must flow at 75 L/s and the oxygen at 100 L/s.Given the geometry assumed above one can calculate flow velocities of 3and 4 m/s.

Given that the calcium oxide will retain some of its heat it is assumedthat a small temperature change of 50 degrees Kelvin exists for thecounter current heat exchanger. If the cast iron surface is treated as auniform wall, one can estimate the heat flux across it. The fluxq″=k/L*ΔT where k is the conductivity, L is the thickness and ΔT is thetemperature change. Solving this equation we obtain a heat flux of 68kW/m². The required heat flux is 850 MJ in one hour or 236 kW whichwould therefore require a surface area of 3.5 m². For the abovementioned cavity, this is a length of 14 m or 18 complete revolutionsaround a screw axis. If it is assumed that one revolution requires threetimes the thickness, then an 8 m long screw is obtained. If it isassumed that a void ratio of 1 and a density of 2.7 Mg per m³ then atotal calcium oxide volume of 0.95 m³ can be calculated. If this volumewere spread evenly over the surface it would be 27 cm thick. All of theabove-mentioned quantities are within the limits of current engineeringpractices. One aspect of the invention is the use of a hollow screw as aheat exchanger.

Laminar Forced Draft Tower. This invention is a novel way of scrubbing avolume of gas in a contained system. The device is a vertical tower withsolid outside walls. The inside of the tower consists of vertical tubesplaced adjacent to each other in a honeycomb like structure. The towerheight would range from 1 to 3 m. The top and bottom of the pipe stackwould be covered with a plate that blocks the annular spaces therebyallowing fluid flow through only the tubes. This system will allow anyliquid sorbent to be distributed above the stack, flow down the insidewalls of the tubes and be collected at the bottom. The gas flow, in ourcase air, will flow up the tower countercurrent to the sorbent. Thediameter of the tubes will be such that the airflow up the stack will belaminar. The air inlet below the tube stack will ring the base of thetower to allow a concentric flow of air up the tower. The airflow inthis tower can be generated by active, fans, or passive means. Oneaspect of the invention is the use of the tower in a forced draft, wetscrubbing system.

Natural Draft Air Flow System. This invention describes ways to move agas through a forced draft system using passive energy sources such asthe sun and the wind. The wind can be harnessed using Venturi devices inorder to create a vacuum at the top of the tower. A venturi consists ofa conical structure that is open at both ends and exposed to the wind.The venture is able to rotate such that larger opening is facing thewind, as the wind passes through the device the physical restrictioncaused by the cone results in an increase in air velocity and a decreasein pressure. A conduit is connected from the base of the venturi to thetower such that this drop in pressure creates a vacuum in the conduitand draws air through the tower. Solar energy can be used to create anatural draft by encasing a volume of air in glass. As the solar energyheats the air in the glass it increases the pressure in the structureand the air will rise. Another conduit is connected from the bottom ofthe glass structure to the top of the tower such that as the air in theglass rises it will draw air through the tower. One aspect of theinvention is the use of a natural draft air flow system in an airscrubbing system.

Small tube coal combustor. The invention proposes a coal combustionsystem that occurs in small tubes using oxygen gas. This system wouldcontain many tubes each less that 6″ in diameter and located on smallangle relative to the horizontal plane. Fine coal particles would be fedinto the elevated end and slowly slide down to the low end. This motionmay be assisted by vibration if necessary. The low end of the tubecontains the oxygen feed, a flame, and an ash collector. The flameignites the oxygen and coal to maintain a steady temperature in thetube. The ash is collected and removed from the system. The tubes can besubmerged in water or another medium in order to transfer the heat ofcombustion to the working fluid. One potential use of these tube bundleswould be in generating steam for power generation while another may beto provide heat to calcite particles for calcination. In one embodiment,the tube is solidly filled, in another embodiment, the tube utilizes afuel injector. Since coal is a very cheap fuel for the calcinationprocess cost effective implementations may use coal for the calcinationstep. However, coal because of its ash content should not be mixed withthe lime that is to be recycled. Consequently, this invention aims todevelop a system of internally heated tubes that provide the heat sourcefor the calcination and transfer heat into the fluidized limestone bedby means of heat exchanger surfaces. One such implementation would betubes that are filled with coal.

Another design would be one where the calcium oxide is moving throughsmall entrained beds that consist of tubes that cross through what lookslike a big conventional boiler. Coal fines are injected with a smallamount of CO2 as a driving gas into a long tube in which coal fines andoxygen mix. One aspect of the invention is the use of a small scale coalfired heat source.

Air Extraction

Mitigating the majority of the anthropogenic CO₂ emissions may require avariety of solutions. This is due to the varied nature of the emissions,both in location and magnitude. Different problems require differentsolutions. Air extraction refers to the removal of gaseous CO2 fromambient air. It produces a stream of concentrated CO₂ ready forsequestration. By its nature, air extraction can capture CO₂ from anysource. A specific implementation the Na⁺/Ca²⁺ process is presentedbelow.

Air extraction enables CO₂ trading by capturing CO₂ anywhere, any timeand from any emitter. It is a stand-alone technology that can be mixedwith other capture schemes allowing separate optimization of energyconversion, capture and storage. It is well suited for: distributedand/or mobile sources, existing infrastructure ill-suited for retrofit,handling leakage from storage sites, challenging CO₂ transportscenarios, driving capture to or above 100%.

In one embodiment, the following chemical reactions are included in themethod:

A. Extraction/Capture from the Atmosphere

2NaOH(aq)+CO₂(g)→Na₂CO₃(aq)+H₂O(l); ΔHo=−109.4 kJ/mol

B. Causticization

Na₂CO₃(aq)+Ca(OH)₂(s)→2NaOH(aq)+CaCO₃(s) ΔHo=−5.3 kJ/mol

C. Calcination

CaCO₃(s)→CaO(s)+CO₂(g); ΔHo=+179.2 kJ/mol

D. Hydration

CaO(s)+H₂O(l)→Ca(OH)₂(s); ΔHo=−64.5 kJ/mol

E. Drying

H₂O(l)→H₂O(g); ΔHvap=+41 kJ/mol 373K,1 atm.

F. Combustion

CH₄(g)+2O₂(g)→CO₂(g)+2H₂O(g); ΔHo=−890.5 kJ/mol

A. Capture from the atmosphere. The overall reaction that takes placeduring absorption in this embodiment of the invention is:2NaOH(aq)+CO₂(g)→Na₂CO₃(aq)+H₂O. The rate equation is:r_(g)=k_(d)[CO₂][OH⁻]. Given that [CO₂] is ˜10 μmol/L at eq., it can beassumed that the reaction is first order as long as the pH is >11. Theeqn is:

J_(CO) ₂ =√{square root over (D_(L)k_(d)b_(o)ρ′_(CO) ₂ )}

for 1 mol/L at 25° C. J=38 μmol/m² s. In this aspect, the absorptionreaches a maximum at 2.5 M. In another aspect, the CO₂ is captured fromthe atmosphere using an alkaline absorber in a wet scrubber according tothe above reaction. The flux produced by this reaction can be estimatedusing kinetic theory 1 and is given by equation (8). In this equation,D_(L) is the diffusivity of CO₂ in water; k_(d) is the kinetic constant;b_(o) is the concentration of hydroxide; ρ′cO2 is the concentration ofdissolved CO₂ at the surface. For a 1 mol/L NaOH solution at 25° C. themaximum flux is 38 μmol/m² s. This value is larger than the publisheddata² ranging from 3 to 10 μmol/m² s. The CO₂ flux is proportional tothe square root of molarity, until it reaches a maximum³ at 2.5 mol/L.This translates to a theoretical flux of 75 μmol/m² s. A scrubber thusrequires about 1000 m² of internal surface per square meter of air flowfor efficient removal.

B. Transfer from Sodium to Calcium Ion.

Na₂CO₃(aq)+Ca(OH)₂(s)→2NaOH(aq)+CaCO₃(s)

The absorbed CO₂ can be removed through precipitation using calciumhydroxide according to the above reaction. The reaction is spontaneous.This reaction is limited by the hydroxide concentration as stablecalcite precipitate will only form in solutions equivalent to 1 to 2mol/L NaOH₄. The process is well documented in the pulp and paperindustry and will reach over 90% completion in less than an hour.Temperature affects both the kinetics of the reaction and the size andtype of precipitate formed. The size of the crystals increase from 5 μmto 15-25 μm as the temperature rises from 25° C. to 75° C. The kineticsand precipitate size can improve with temperature. The increasedparticle size can improve filtration.

C. Separation Calcite Drying and Hydrating.

CaO(s)+H₂O(g)→Ca(OH)₂(s)

Given the upper limit of 1 mol/L NaOH, the emulsion resultant from thecausticization step will contain approx. 5% solids. Filtration, for asimilar particle size, can produce calcite filter cakes with a moisturecontent of 19.5% under a vacuum of 96 kPa. This is close to the optimumfilter cake moisture content for this process. The heat required toevaporate water will be obtained from the hydration reaction shownabove. This reaction produces 2.4 GJ of heat per tonne of CO₂ but islimited by the dehydration temperature of 420° C. In order to generatesufficient steam for the hydration reaction from the drying step, thecalcite filter cake would require a moisture content of 15%. Onesuitable method is steam drying. The calcinations step is a maturetechnology with minimal energy penalty of 4.1 GJ/tonne CO₂. Modern kilnsget 90% efficiency, and a flash calciner gets 70%. Conversion tooxy-fuel would raise it the efficiency to 80% or 5.13 GJ. In one aspectof the invention, Solar Thermal Ovens achieve 98% conversion for kg/hr.

D. Regeneration Via Thermal Decomposition.

CaCO₃(s)→CaO(s)+CO₂(g)

The calcination of the limestone returns the CO2 to gaseous form in aconcentrated stream. The thermal decomposition of calcite occurs at˜900° C. in pure CO₂ (pCO2=1 atm.) according to the above equation. Thisprocess is commercially mature and for the given particle size of 5-25μm, a flash calciner or fluidized bed would be suitable. The theoreticalenergy penalty for the calcination reaction is 4.1 GJ/tonne of CO₂ whilethe flash calciner operates at ˜5.87 GJ/tonne of CO₂ or at 70%efficiency. The required heat input can is obtained by the combustion ofcoal gas or methane while the resultant CO₂ can be captured by usingpure oxygen instead of air as the O₂ source. The high CO₂ content of thereactor gas will inhibit the reaction kinetics. This effect can becountered by raising temperature or introducing steam into the reactor.

5. Air Capture Device. Given the atmospheric CO₂ concentration of ˜380ppm or 0.015 mol/m3, removing one tonne of CO₂ per hour will require 2.3million cubic meters of air, assuming 66% removal. At an airflow speedof 2 m/s one requires a frontal area of 18 m by 18 m. Scrubber surfacesinternal to the device are about a factor of 1000 larger. This can beachieved with a variety of packing. A forced draft, packed tower using aRauschert™ Hi-Flow Rings (313 m2/m3) would require a volume of 2000 m³or ˜30 towers of 10 m height and 3 m diameter. If the absorption ratewere raised to the physical limit near 75 μmol/m² s then the number oftowers would be reduced to 4. Slow wind speeds maximize contact timewhile minimizing the loss of kinetic energy. Assuming the ideal systemoperates at the edge between liquid and airside limitations in a liquidside limited regime, one can calculate the air side boundary layerthickness (δ) using Fick's Law of Diffusion. Using the 1 mol/L solutionwe obtain an air side boundary layer thickness of 6 mm. A boundary layerof this thickness allows for operation in the laminar regime. Togetherwith the desire to keep the capture system as compact as possible, thisdetermines the geometry of the system.

6. Process Options to Increase Efficiency.

Highly Caustic Solutions—Wet scrubbing results in the loss of watervapor entrained in the exit gas. As a result, make up water will berequired and may limit the site selection if cost and supply areunfavorable. The amount of water loss can be controlled by manipulatingthe NaOH concentration of the absorbing solution. Water losses areeliminated when the hygroscopic solution is in equilibrium with theambient relative humidity.

Solar Oven The calcination reaction is the most energy intensive portionof the process. Reducing the amount of fuel consumed would decrease thecost per tonne of CO₂ significantly. A solar rotary kiln can produce 98%purity lime at a production rate of kg's per hour. If this technologycan be scaled up to tonnes per hour then it may be suitable for airextraction. Additionally, the use of calcite as a refractory liningwould increase the efficiency of the flash calciner proposed.

Hot Causticization As stated above, the higher the temperature of thecausticization reactor the larger the precipitate and the more efficientthe filtration. Any attempt to heat the solution would require ˜10 GJ ofenergy. The large volume of water (48 m³) does, however, provide anexcellent heat sink for all waste heat that is produced in the process.Even if it falls short of 10 GJ, it nevertheless improves the propertiesof the precipitate.

Filter Aids Filter aids are added to the precipitate in order tomaintain pore size, make the cake less compressible, and provide fasterfiltration.

1. A system for extracting or capturing carbon dioxide from atmosphericair, comprising: adjacent towers each including a front portion, sideportions joined with said front portion and extending to end edges, andan open back portion defined between said end edges, said tower portionsincluding front surfaces and back surfaces, said towers positioned so asto define a narrowed passage therebetween; a wet scrubbing mechanismpositioned within each open back portion between said end edges, saidwet scrubbing mechanism introducing a basic solution to absorb at leasta portion of an amount of carbon dioxide contained in atmospheric airflowing through each open back portion thereby forming a carbonatesolution; a filter positioned on said back surfaces; adjustable openingsdefined in side portions of said towers that are adjacent to saidnarrowed passage, said openings providing a conduit between said frontand back surfaces via said filter; a treatment module including a supplyof reagent in communication with said carbonate solution forcausticizing said carbonate solution; and a regeneration module forhydrating solid components remaining after said treatment module toregenerate a base material included in said basic solution.
 2. Thesystem according to claim 1, wherein said narrowed passage is configuredto cause atmospheric flowing therein to reduce in pressure therebydrawing atmospheric air located in said open back portion and having ahigher pressure through said filter and into said narrowed passage. 3.The system according to claim 1, wherein said adjustable openings aredefined by shutters, baffles, or the like.
 4. The system according toclaim 1, further comprising a supply of basic solution in fluidconnection with said wet scrubbing mechanism.
 5. The system according toclaim 4, wherein the base material included in said basic solution isselected from sodium hydroxide, calcium hydroxide, and potassiumhydroxide.
 6. The system according to claim 5, wherein the base materialis sodium hydroxide.
 7. The system according to claim 1, wherein thecarbonate solution is a sodium carbonate (Na2CO3) solution.
 8. Thesystem according to claim 1, wherein said supply of reagent is sodiumtri-titanate or calcium hydroxide.
 9. The system according to claim 1,wherein said openings are increased in size when a high speed wind ismoving through said narrowed passage and decreased in size when a lowspeed wind is moving through said narrowed passage thereby maintaining asubstantially constant pressure drop across said filter.
 10. The systemaccording to claim 1, wherein said treatment module further comprises aheat source for heating or calcining said carbonate solution.